In a continuously variable transmission, a ratio-varying unit (“variator”) is used to provide a continuous variation of transmission ratio. Variators, and specifically toroidal variators, are used to provide a continuously variable transfer rotation from an input to an output. A full toroidal variator includes two output races each having a partial toroidal cross-sectional profile. The races are configured with their toroidal profiles exposed oppositely with a rotational output secured between the two races so that the two races and rotational output rotate together about a common axis. The two output races cooperate to define a toroidal output that is normally supported on a rotating shaft, with the toroidal output supported on the shaft by roller bearings that permit the toroidal output to rotate relative to the rotating shaft.
The full toroidal variator generally also includes a pair of input races positioned on the rotating shaft so that each input race is opposite one of the output races so that an input race and an output race cooperate to define a cross-section of a torus. The input races are keyed to the rotating shaft such that they rotate with the shaft during rotation of the shaft. A full toroidal variator typically has a number of rollers that are positioned between the respective sets of input races and output races, the rollers transferring rotation of the input races to the output races. The rollers are pivotably supported in a manner that allows their orientation to be altered to change the ratio of input to output through the variator.
Generally, there is no direct contact between the rollers and the races and the motion transferred therebetween is accomplished by transferring friction between a race and a roller through a generally incompressible fluid medium that is subjected to shear during rotation of the input races. The shear applied to the fluid medium acts on the roller surface proximate the input races, causing the rollers to rotate about their axis. The rotation of the roller is transferred to the output races in a similar manner with rollers applying shear to the fluid medium that causes motion to be transferred to the output races. Such an arrangement eliminates the need for direct contact between the rollers and races, thereby reducing the opportunity for galling of the rollers and races and providing for longer life of the variator.
Because the variator is used to transfer torque as well as motion, it is important to maintain the fluid medium at sufficient pressure to cause the motion to be transferred between the races and rollers with minimal slip. Pressure between the rollers and races is applied through an endload arrangement that operates to increase the pressure in the variator when increased torque conditions occur. The load is applied to one of the input races in an axial direction that varies a clamping force between the input races and the output races.